The atmosphere has a fixed water-holding capacity, dependent on its temperature at any time. When the water content in the air is equal to this capacity, the air is saturated.

A is incorrect. Relative Humidity refers to the amount of water content in the air, as compared to its saturated counterpart.

B is incorrect. Specific Humidity refers to the amount of water vapour present in the air per gram of air mass.

C is incorrect. Absolute humidity refers to the amount of water vapour present in the air per metre cube, regardless of the temperature of the air.